Fig. 5. Fine mapping of the P1 nuclease-hypersensitive sites on the G–rich and C–rich DNA strands within the R–loop formed at an 832 bp fragment of the Sγ2b sequence. (A) See legend of Figure 4 for description of gels. Ladders of 50 and 100 bp were used as standards. The start of Sγ2b is shown on the left panel. The border between the single-stranded and double-stranded regions of the G–rich strand is indicated by the open arrow. (B) Putative model for the R–loop structure formed at a portion of the Sγ2b sequence as inferred from the P1 nuclease probing data. In the text, we explain why the G–rich RNA strand only forms a stable hybrid with a portion of the C–rich (template) DNA strand.